Envelope detectors are commonly used for demodulating low-frequency information content from higher-frequency carrier signals. An example of such an application is in RFID chips, for example, where circuitry is required to extract amplitude modulation envelope information from received radio-frequency signals.
In RFID chips, envelope detectors can be used to convert the amplitude modulated signal sent by a tag reader into a so-called baseband signal which contains the modulation signal, without the radio frequency carrier signal present in the reader signal. Such a signal can then be exploited by the logic portion of the RFID chip.
Full-wave envelope detector circuits have been used in RFID chips, but typically suffer from disadvantages including poor sensitivity (ie they are unable to operate with low signal amplitudes), variations in manufacturing process, instability due to temperature drift and/or a variation in the output signal when the RF power of the input signal varies.
It has been suggested in patent document U.S. Pat. No. 4,000,472 to address some of these problems, and in particular the problems of poor sensitivity and temperature drift. In the circuit proposed in U.S. Pat. No. 4,000,472, a voltage doubler circuit, forming an amplitude modulation envelope detector, is forward biased using a forward biasing circuit. This biasing circuit comprises a current source formed by a resistor, which provides a bias current in order to increase the linear dynamic range of the voltage doubler. The temperature drift is compensated by means of a temperature compensation circuit associated to an analog-to-digital converter formed by an operational amplifier. This compensation circuit also receives the bias current. The voltage doubler circuit has two diodes whose electrical characteristics vary with varying temperature, and the temperature compensation circuit has two further diodes, matched to the two diodes of the voltage doubler circuit, which provide a compensation reference to the inverting input of the operational amplifier. The circuit proposed in U.S. Pat. No. 4,000,472 is relatively complex and makes use of resistive elements and an operational amplifier. As such, the solution does not lend itself to miniaturized or very low power applications such as RFID tags. Resistive components require significant silicon real-estate and may increase power dissipation, and the inclusion of an amplifier circuit also increases size, complexity and power consumption. Further, the temperature compensation is not done at the level of the envelope detector, but at a higher level involving an operational amplifier.
Another disadvantage with some prior art envelope detectors is that there is a DC voltage on their output even when no input signal is present, and this DC voltage must preferably be compensated for.